I guess you should read the microsoft specs.
Easiest way is to get Windows 2003 R2 64-bits!! (Latter part is mandatory!) with all patches and install exchange 2007 SP1. (Saves you updating).
I don’t know if you should use the older version of Windows 2003.

Be sure, if you want to migrate from exchange 2003 that that should be fully patched, otherwise you can’t create a mail connector between the two sites.